1. Field of the Invention
The subject invention relates to the manufacture of polycaprolactam. More particularly, the subject invention relates to a process for increasing the molecular weight and simultaneously removing unreacted monomer and oligomers from the unextracted polycaprolactam by treatment with hot inert gas having a specific moisture content. This treatment is herein referred to as "tempering". The tempered polycaprolactam granules, when prepared by the process of the invention, are suitable for further melt processing, e.g. in spinning carpet yarns.
2. Description of the Related Art
Polycaprolactam is produced by the polymerization of caprolactams, principally, .epsilon.-caprolactam, optionally in the presence of other amide-linkage-producing monomers. While a number of polymerization initiators may be utilized, water still remains the initiator of choice. The presence of small quantities of water in the .epsilon.-caprolactam monomer causes hydrolysis of a portion of the .epsilon.-caprolactam to .epsilon.-aminocaproic acid, which then initiates the ring opening polymerization resulting in the formation of polycaprolactam.
The polymerization reaction eventually reaches a steady state equilibrium in which the product contains significant amounts of water, .epsilon.-caprolactam monomer, caprolactam-derived oligomers, and polymer. When high molecular weight products are desired, the polycaprolactam prepared as described, is extracted with organic solvents and/or water to remove unreacted monomer and oligomers, followed by drying in hot inert gas or under vacuum. Alternatively, monomer and some oligomers can be removed by vacuum demonomerization of the melt.
When lower molecular weight products suitable for melt-spinning of fibers are desired, chain regulators are often added to the reaction mixture to limit the molecular weight to lower values. Suitable chain regulators are monoamines or diamines, preferably the former, and mono and dicarboxylic acids, again with preference being given to monofunctional chain regulators. The chain regulated polymers also are washed with organic solvent and/or water to remove extractable components and then dried, or melt vacuum demonomerized.
U.S. Pat. No. 3,155,637 discloses a process for removing extractables and simultaneously increasing the molecular weight of polycaprolactam which has been prepared in the absence of chain regulators, by subjecting the unextracted granules to treatment with superheated steam, optionally containing an inert gas such as nitrogen or carbon dioxide. When chain regulated polycaprolactam is tempered by this process, however, the product produced does not have the dyeability characteristics nor the stable high temperature resin viscosity required for spinning fibers suitable for carpet and other yarn applications. Moreover, at high moisture levels, hydrolysis of polycaprolactam into monomer and oligomers occurs and resin viscosity may decrease rather than increase.